Dear Nagaseeker,
Its about one's intention isnt it?
When we walk out our door, I don't think we look for insects etc to step on right?
If we are sincere about not contributing towards animals being slaughtered for food, and we consciously buy free range age , for most of us that is as sincere as we can get. Since we are aware of not harming animals ,and had taken measures to avoid doing that, we will substantially avoid the full effect of any negative action even if insects etc are killed by our walking or the eggs turned out to be produced from factory.
We can not get a 100 % score in this karma created planet even with our best intentions and actions. We are doing great if we don't get a F mark which is intentionally causing harm.
In the scenario of khata offering,if u met a holy Lama without yr own cotton khata, and is offered a silk one, I don't think you are going to incur much karma . The most important factor which decides whether we will experience the full result of a negative action is one's intention. When this is not present, most of our actions will not have enough power to ripen as heavy negative karma. The negative potency is so little that it could only ripen as a slight headache the next morning.
The lamrim chapter on karma under the sub topic - the factors of completion of any action, and the severity of any actions is a great help to figuring out these sort of to do or not to do questions.